1. Field of Invention
The present invention relates to crush-resistant flexible tubing for conveying gases. The tubing is formed of elastomeric material that initially is in an uncured condition and, when formed and cured, provides a strong but flexible length of tubing. More particularly, the invention relates to tubing that has helical corrugations along its length and also, relates to a method for making the tubing.
2. Description of Related Art
Corrugated tubing of the general type to which the invention relates is both flexible and strong, yet still retains its tubular form in a semi-rigid condition. This is accomplished by providing the tubing with helical corrugations. This type of tubing is used in a variety of home and industrial applications, such as for vacuum cleaning systems, engine exhaust systems for automotive service facilities and for numerous other purposes.
In accordance with prior practice, helically corrugated tubing may be made by first sliding an extruded sleeve of uncured rubber axially over a rotatable mandrel, the mandrel having a continuous thread formed on its outer surface. When in place on the mandrel, the sleeve is forced into the helical groove or root of the thread by a length of cord wrapped around the sleeve as the sleeve rotates with the mandrel. This serves to impart a desired corrugated shape to the uncured rubber sleeve.
The resulting assembly is then removed from its rotary support preparatory to curing. In this condition the rubber sleeve is cured in an oven or autoclave to set the helical corrugations. The result is a cured length of rubber tubing with helical convolutions.
The cord is removed from the corrugated tubing by placing the assembly back on a rotary support and rotating the mandrel in the reverse direction while unwinding the cord from the cured tube. After the removal of the cord, the corrugated tube length may be removed from the mandrel by introducing air under pressure between the outer surface of the mandrel and the inner surface of the corrugated tube. The overall method described above is generally known as the “cording” process and is shown and described in more detail in U.S. Pat. No. 2,832,086. The process of removing the cured tube from the mandrel using pressurized air is shown and described in U.S. Pat. No. 2,888,712.
The resulting tubular product with helical convolutions is both flexible and resilient but rigid enough to resist crushing such as when the tube is stepped on. Also, the tube has wall with a generally uniform thickness, the wall defining continuous internal and external threads with alternating crests and roots along the tube length. In other words, the wall portion that defines a crest portion of the external thread also defines on its opposite side, the root portion of the internal thread.
As viewed in longitudinal or axial section, the wall has a sinusoidal form with alternating crests and roots as to both the internal and external surface portions. Both the internal and external crests are rounded or semi-circular as viewed in longitudinal section and have about the same shape (i.e. about the same radius). Likewise, both the internal and external roots are rounded or semi-circular as viewed in longitudinal section and have about the same shape (i.e. about the same radius). However, the radii of the external and internal crests are greater than the radii of the respective roots, the difference being about equal to the thickness of the tubular wall.
Also, in past practice, the sinusoidal shape of the wall as viewed in axial section is such that the center of the semi-circular crest shape for the outer surface has about the same radial distance from the axis of the tube as the center of the semi-circular crest shape for the inner surface. In order to achieve this characteristic for the tubing, the thread has a very small pitch that is achieved by having a minimal spacing between the convolutions of the cord when the tube is formed on the rotating mandrel. These related characteristics result in a high resistance to axial compression of the tube. At the same time, these characteristics produce a high resistance to crushing.
While resistance to axial compression is desirable for many applications, there are some circumstances where it would be desirable to be able to axially compress crush-resistant flexible tubing for conveying gases with minimal force and retain it in a collapsed condition (e.g., for storage and transport). One application where axial shortening or compression of the corrugated tubing would be beneficial, if not essential, is in the case of mobile military field equipment. In modern combat operations, it is necessary to be prepared for chemical and/or biological attack. With certain current types of biochemical weapons, gas masks alone would not protect against casualties. In the case of some biochemical agents, any contact with the body could disable the soldier.
Accordingly, to protect against biochemical warfare, military vehicles must have air purification equipment to provide safe air to the vehicle's occupants. Also, sealed enclosures (e.g. tents) with air purification capability are needed. To achieve this, it is necessary to have means to connect either a vehicle's own air purification system or a separate mobile air purification unit to an adjacent sealed enclosure. Flexible corrugated tubing of the general type described above is ideally suited for this purpose, provided it may be axially compressed with minimal force to reduce its size for storage and transport. This is particularly important in the case of combat vehicles and other mobile combat equipment because space is at a premium.
Accordingly creative modifications to the design and production of helically corrugated tubing using the “cording” process have been developed to meet the requirements described above. The tubing and process for making it as embodied in the present invention achieve these results and afford other features and advantages heretofore not obtainable.